SUMMARY/ABSTRACT It is estimated that between 1.3 and 2.1 million youth in the U.S. experience homelessness each year. While cigarette smoking among youth in general continues to decline in the U.S., approximately 70% of unaccompanied homeless youth are current cigarette smokers. In addition to smoking cigarettes, 72% of these homeless youth smokers use alternative tobacco products (ATPs) such as e-cigarettes, cigarillos, hookah, and chewing tobacco or other smokeless tobacco products ? a rate which is substantially higher than national estimates for adolescents and young adults. ATP use is associated with a range of health problems, and may be particularly problematic for homeless youth given the health threats they already face due to harsh living environments, poor nutrition, mental disorders, substance abuse, and inadequate access to health care and prevention services. Unfortunately, there are no published studies of ATP use among homeless youth to guide efforts to prevent or reduce the use of these products. This project will address a critical gap in the literature on youth tobacco use by focusing on ATP use among homeless youth, as well as provide the type of comprehensive information on ATP use that is necessary to inform programs aimed at preventing and reducing the use of these products. What is currently known about ATP- related attitudes and behaviors among adolescents and young adults primarily comes from school-based samples, and we cannot assume that existing findings generalize to ?street youth? who are homeless and living on their own. The goals of this project will be achieved through three interrelated aims that utilize multiple data sources and mixed methods to achieve a comprehensive understanding of how to best reduce all forms of tobacco use among homeless youth. Aim 1 involves collecting qualitative focus groups data from homeless youth to better understand of their: (a) knowledge of different types of ATPs; (b) experiences with and future intentions to use ATPs; (c) motivations to use ATPs and perceived pros/cons of specific ATPs; and (d) access to ATPs and exposure to ATP- related advertising and marketing. Informed by the Aim 1 findings, Aim 2 involves collecting quantitative survey data from a probability sample of 300 homeless youth to: a) investigate the prevalence and patterns of ATP use; b) compare perceived availability, affordability, and harm between various ATPs and cigarettes; and c) identify the correlates of current ATP use, intentions to use in the future, and quitting. Aim 3 involves developing recommendations for environmental policy and cessation treatment strategies, addressing both cigarette and ATP use among homeless youth, based on key findings from this project and a recent project by our team on cigarette smoking among homeless youth. We will obtain feedback on these recommendations, through interviews with service providers and focus groups with homeless youth, and disseminate a final report to key stakeholders.